1. Field of the Invention
The invention relates to a method for adjusting dot forming or depositing positions in dot matrix recording and a printing apparatus using the method. More particularly, the invention relates to a method for adjusting dot forming positions, which are applicable to printing registration in the case of bi-directionally printing by a forward and reverse scan of a print head or to printing registration in the case of printing by means of a plurality of print heads, and printing apparatus using the method.
2. Description of the Related Art
In recent years, office automation instruments such as the personal computer and the word processor, which is relatively cheap, are widely used, and an improvement in high-speed technique and an improvement in high image quality technique of various recording apparatuses for printing-out the information which are entered by the instruments are being developed rapidly. In recording apparatuses, a serial printer using a dot matrix recording (printing) method is a recording apparatus (a printing apparatus) which realizes printing with high speed or high image quality but with low cost. For such printers, which print at high speed, for example there is a bi-directional printing method, as well as which print in high image quality, for example, there is a multi scanning printing method.
(Bi-directional printing method)
To improve high-speed printing, in a printing head which has a plurality of printing elements (although it is also considered to increase the number of a printing elements and improve a scanning speed of the print head), bi-directional printing scans of the print head are performed.
Although, since there is usually the time required for paper-feeding and paper-discharging or the like, it does not become a simply proportional relation, in the bi-directional printing, a printing speed of approximately two times can be obtained as compared with the one-directional printing in the printing apparatus.
For example, when using the print head having 64 ejection openings arranged with 360 dpi (dots/inch) in printing density in a direction different from the printing scanning (main scanning) direction (for example, in a sub-scanning direction which is the feeding direction of the printing medium), a printing is performed on, a printing medium of A4 size set in the lengthwise direction, the printing can be completed by scanning approximately 60 times. The reason is that, in one-directional printing, each printing scanning is performed only at the time of the movement in the one direction from the predetermined scanning commencement position, and since non-printing scanning to the inverse direction for returning to the scanning commencement position from a scanning completion position is attended, reciprocation of approximately 60 times is required. On the other hand, printing is completed by reciprocating printing scanning of approximately 30 times in bi-directional printing, so that printing can be performed and since it becomes possible at a speed of approximately 2 times. As such, bi-directional printing can be considered to be an effective method for an improvement in a printing speed.
In order to register dot-forming positions (for example, for an ink jet printing apparatus, a deposition or landing position of ink) at a forward trip and a return trip together in such bi-directional printing, using a position detection means such as an encoder, based on the detecting position, printing timing is controlled. However, to form such a feedback controlled system causes an increase in the cost of the printing apparatus. As a result, it is difficult to realize this in a printing apparatus which is relatively cheap.
(Multi scanning printing method)
Secondly, a multi scanning printing method is explained as one example of an improvement in high image quality technique.
When printing is performed using a print head which has a plurality of printing elements, quality of the printed image depends on performance of a print head itself greatly. For example, in the case of the ink-jet print head, slight differences, which is generated in a print head manufacturing step, such as variations of a form of ink ejection openings and the elements for generating energy for ejecting ink such as an electro-thermal converting elements (ejection heaters), influence a direction and an amount of ejected ink, and result in an unevenness in density of the image which is formed finally thereby reducing the image quality.
Specific examples are described using FIGS. 1A to 1C and FIGS. 2A to 2C. Referring to FIG. 1A, a reference numeral 201 denotes a print head, and for simplicity, is constituted by eight nozzles 202 (herein, as far as not mentioned specifically, refer to the ejection opening, the liquid passage communicated with this opening and the element for generating an energy used for ink). A reference numeral 203 denotes the ink, for example, which is ejected as a drop from the nozzle 202. It is ideal that the ink is ejected from each ejection opening in an approximately uniform amount of discharge and in a justified direction as shown in this drawings. When such discharge is performed, as shown in FIG. 1B, ink dots which are justified in size are deposited or landed on the printing medium and, as shown in FIG. 1C, the uniform images are produced with no unevenness in density as a whole can be obtained.
However, there are the variations in the nozzles in the print head 20 actually as is mentioned above, and when printing is performed as mentioned above, as shown in FIG. 2A, the variations are caused in size of the ink drops and in the ejecting direction of ink discharged from nozzles and the ink drops deposited or landed on a printing medium as shown in FIG. 2B. In this drawing, part of the white paper there exists an area factor can not be served up to 100% periodically with respect to the horizontal scanning direction of the head, moreover, in contrast with this, the dots overlap each other more than required or white stripes as shown in the center of this drawing have been generated. A gathering of the landed dots in such condition forms the density distribution shown in FIG. 2C to the direction in which nozzles are arranged, and the result is that, so far as usually seen by eyes of a human, these objects are sensed as the unevenness in density.
Therefore, as a countermeasure of this unevenness in density, the following method has been devised. The method is described using FIGS. 3A to 3C and FIGS. 4A to 4C.
According to this method, in order that the printing with regard to the same region as shown in FIGS. 1 to 1C and FIGS. 2A to 2C is made to be completed, the print head 201 is scanned 3 times as shown in FIG. 3A and FIGS. 4A to 4C. The region defining four pixels which is a half of eight pixels as a unit in the direction of length in the drawing has been completed by two passes. In this case, the 8 nozzles of the print head are divided into a group of 4 nozzles of upper half and 4 nozzles of lower half in the drawing and the dots which one nozzle forms by scanning of one time are the dots that the image data are thinned into approximately a half in accordance with the certain predetermined image data arrangement. Moreover, at the second scanning, the dots are embedded in the image data of the half of the remaining and the regions defined four pixels as the unit are completed progressively. Hereinafter, the printing method described above is referred to as a multi scanning printing method.
Using such printing method, even when the print head 201 which is equal to the print head 201 shown in FIG. 2A are used, the influence to the printed image by the variations of each nozzle is reduced by half, whereby the printed image becomes as shown in FIG. 3B and no black stripe and white stripe as shown in FIG. 2B becomes easy to see. Therefore, the unevenness in density is fairly also mitigated as compared with the case of FIG. 2C as shown in FIG. 3C.
When such printing is performed, although at a first scanning and at a second scanning, the image data are mutually divided in a manner to be complementary to each other in accordance with a certain predetermined arrangement (a mask), usually, this image data arrangement (the thinned patterns) as shown in FIG. 4A to FIG. 4C, at every one pixel arranged in rows and columns, it is most general to use the formation which makes to form a checker or lattice matrix. In a unit printing region (here, four pixels), printing is completed by the first scanning which forms the dots into the checker or lattice pattern and the second scanning which forms the dots into the inverted checker or lattice pattern. Moreover, usually, travel (vertical scanning travel) of the printing medium between each main scanning is established at a constant, and in the case of FIGS. 3A to 3C and FIGS. 4A to 4C, is made to move every four nozzles equally.
(Dot alignment)
As an example of other improvements in high image quality technique in the dot matrix printing method, there is a dot alignment technique adjusting the dot depositing position. A dot alignment is an adjustment method adjusting the positions which the dots on the printing medium have formed by any means, and in general, the prior dot alignment has been performed as follows.
For example, a ruled line or the like is printed on a printing medium in depositing registration of the forward scan and the reverse scan upon reciprocal or bi-directional printing by adjusting printing timing in the forward scan and the reverse scan respectively, while a relative printing position condition in reciprocal scan is varied. The results of printing has been observed by a user oneself to select the printing condition where best printing registration is achieved, that is, the condition that printing is performed without offset of the ruled line or the like and to set the condition directly into the printing apparatus by entering through a key-operation or the like or to set the depositing position condition into the printing apparatus by operating a host computer through an application.
Moreover, the ruled line or the like is printed on the medium under printing in the printing apparatus having a plurality of heads, when printing is performed between a plurality of heads, while a relative printing position condition between a plurality of heads is varied, with the respective head. As is mentioned above, the optimum condition that best printing registration is achieved has been selected to vary the relative printing position condition to set the printing position condition into the printing apparatus every each head in the mentioned-above manner.
Here, the case where the offset of the dots has been occurred is described.
(Problems upon performing image-formation by bi-directional printing) Due to bi-directional printing, the following problems has been caused.
First, when the ruled line (the ruled line of the longitudinal direction) in the direction perpendicular to the horizontal scan of the print head is printed, between the ruled line element which is printed in the forward scan and the ruled line element which is printed in the reverse scan, the dot depositing positions are not registered and the ruled line is not formed into a straight line, but rather a difference in level occurs. This is referred to as a so-called "offset in ruled line", and this is considered to be the most general disorder which can be recognized by the usual users. In many cases, the ruled line is formed by a black color, however, though the offset in the ruled line has been understood as the problem where a monochrome image is formed generally, a similar phenomenon can be caused in the color image also.
When multi scanning printing is used along with bi-directional printing in order to improve in high image quality, even though in bi-directional printing the depositing positions are not registered, as an effect of the multi scanning printing the offset in the pixel level is not easy to be seen, but from a macroscopic viewpoint the entire image can be seen unequally and is recognized as an unpleasant figure by the user. This generally is called texture, and appears on the image in the specific period where there is the offset in the delicate depositing position, thereby being caused. In a strong image in contrast such as the monochrome it is easy to be seen, moreover, when for the printing medium capable of high-density printing such as a coat paper middle-tones printing is performed, it can be easily seen.
(Problems in the case of performing the image formation using a plurality of the print heads)
In the printing apparatus having a plurality of heads, the problems of the case where the offset in the depositing positions of the dots between a plurality of heads occurs is discussed below.
When the image printing is performed, several colors are combined to perform the image formation frequently, and it is general to use four colors which added black in addition to three primary colors of yellow, magenta and cyan and it is used most abundantly. When in the case where a plurality of print heads for printing these colors are used, there is the offset of the depositing positions between the print heads, depending upon the amount of the offset, when a different color one another is about to be printed on the same pixel, a deviation in color matching is caused. For example, magenta and cyan are used to form the blue image, and although the part that the dots of both colors are overlapped becomes blue, the part which is not overlapped each other does not become blue, so that the deviation in color matching (irregular color) that each independent color tone appears is caused. When this occurs partially, it does not become easy to be seen, but when this phenomenon occurs in the direction of scanning continuously, a band-shaped deviation in color matching with a certain specific width is caused, so that the image becomes unequal. In addition, in a region adjacent the image region in the case of in the regions of the same color, when there is no offset in the depositing positions of the dots, a uniform impression and color development differ between the image regions adjacent each other, so that the image that there is a sense of incongruity as the image is formed. Moreover, though this deviation in color matching does not become easy to be seen in the case of an ordinary paper, it becomes easy to be seen, when a favorable printing medium in color development such as a coat paper is used.
Moreover, in the case where a different color is printed on adjoining the pixel, when there is the offset in the depositing positions of the dot, the clearance, that is, the region which is not covered by the ink, the ground of the printing medium can be seen. This phenomenon frequently is called "white clearance", since the case of a white ground is frequent in the printing medium generally. This phenomenon is easy to be seen in an image high in contrast, and when a black image is formed as a colored back ground, the white clearance which no ink is deposited between a black and coloration, since a contrast between white and black is high, can be easily seen.
It is effective to perform the above-mentioned dot alignment in order to suppress the occurrence of the problems as mentioned above. However, the complicatedness that the user should observe the results which the depositing registration conditions are varied by the eyes to select the optimized the depositing registration condition to perform entering operations is accompanied, and moreover, since fundamentally, a judgment for obtaining the optimum printing position by observing through eyes is enforced on the user, the establishment which is not optimized can be set. Therefore, it is especially unfavorable to the user who is not accustomed to operation.
Moreover, the user spends time and effort at least two times since the user should print the image to perform the depositing registration and in addition, to perform conditional establishment after observing to perform judgments required, whereby upon realizing the apparatus or a system excellent in operability, it is not only desirable but also is disadvantageous from the viewpoint of time-consumption.
Namely, it has been desired strongly that the apparatus or system capable of printing the image at a high speed and with high-quality without occurring the problem on the image formation as mentioned above and the problem on the operability is realized at a low cost by registering the depositing position without using a feedback controlling means such as an encoder by an opened loop.
And more particularly, as many of recent printing apparatuses provide an operation mode for performing a printing where a rapid output has priority over the image quality, or provide the ability to select an operation mode for printing with a high image quality at the expense of low output speed, it is desirable to perform simply and rapidly an appropriate dot alignment according to these respective modes.